Helicopters are well known to be inherently unstable and therefore difficult to fly, especially in conditions of degraded visibility and when the air is turbulent. Large helicopters employ force-feel systems and stability augmentation systems (SASs) to improve flying qualities and thereby simplify the flying task. Force-feel systems and SASs are not offered on smaller helicopters because they are complex, heavy, and expensive.
FIG. 1 illustrates a conventional helicopter system 10 with a SAS that includes a force-feel system. The SAS includes a series servo-actuator 12, a trim-motor 14, a feel-spring 16, and a linkage 18 that is inserted between the pilot's cockpit-flight-controller 20 and the flight control surface 22. Thus, the SAS is implemented in series with the flight control system.
The forces felt by the pilot's hand on the cockpit-flight-controller 20 depend on the feel-spring 16 and trim-motor 14. The output of the trim-motor 14 controls the reference point for the feel-spring 16. The pilot controls the output of the trim-motor 14 by using a trim-switch (not shown) typically located on the cockpit-flight-controller 20. The input to the flight control surface 22 includes the sum of the position of the cockpit-flight-controller 20 via a link 24 and the series servo-actuator 12 via a link 26.
Signals transmitted by the SAS (i.e., SAS inputs) to the series servo-actuator 12 include angular rate and/or attitude feedbacks that are intended to damp or stabilize the motions of an unstable helicopter. The effect of the SAS inputs to the series servo-actuator 12 are transmitted to the flight control surface 22 via the mechanical linkages 18 and 26. The linkage 18 is designed so that SAS feedbacks are not felt by the pilot at the cockpit-flight-controller 20. The absence of any forces on the cockpit-flight-controller 20 due to the SAS inputs to the series servo-actuator 12 is an objective of the series implementation. The need for a complex flight control linkage 18, a trim-motor 14 and a feel-spring 16 make the series implementation of a force-feel system and SAS unsuitable for use on light helicopters due to excessive weight and cost.
It is difficult and costly to retrofit an existing helicopter with a conventional force-feel system or SAS. As illustrated by the linkage 18 in FIG. 1, conventional systems require that apparatus be inserted into a break in the existing flight control system. Unless a helicopter already has a series servo-actuator 12 and linkage 18 installed, this is a significant and costly modification. Typically, a series servo-actuator 12 and linkage 18 are found only in large and complex helicopters. For these reasons, conventional systems are not deemed suitable for installation in light helicopters.
FIG. 2 illustrates a conventional autopilot system 28 used on helicopters and airplanes. Compared to the system 10 shown in FIG. 1, the conventional autopilot system 28 utilizes a less-complex parallel servo-actuator 30 implementation. The servo-actuator 30 is mechanically attached directly to and in parallel with an existing flight control system without any modification to the linkage 24 between the cockpit-flight-controller 20 and the flight control surface 22. The servo-actuator 30 includes a gearing device and clutch assembly 32 and an electric motor 34. When the autopilot is engaged, the servo-actuator 30 moves the flight controls 20, 22, 24 to achieve a response commanded by an autopilot computer 40. High gear ratios are employed in the gearing device and clutch assembly 32 to reduce the size of the electric motor 34. The gearing device and clutch assembly 32 exhibits moderate freeplay in typical autopilot systems.
The combination of high gearing and freeplay results in heavy cockpit flight control forces and undesirable looseness in the controls if the pilot attempts to override the autopilot. Thus, the combination results in very objectionable handling qualities if the pilot attempts to control the vehicle manually with the autopilot engaged. Therefore, conventional force-feel systems and SASs designed to augment manual control employ the more complex series mechanization illustrated in FIG. 1.
The conventional autopilot system 28 shown in FIG. 2 may include an override spring (not shown) between the autopilot servo-actuator 30 and the cockpit-flight-controller 20. The purpose of the override spring is to allow the pilot to make minor corrections to the aircraft attitude or flight path without disengaging the autopilot, and to return to automatic flight without having to reestablish trim. The force vs. deflection gradient of the override spring must be sufficiently large so that the autopilot servo-actuator 30 can drive the cockpit-flight-controller 20 without introducing additional dynamics due to the override spring. However, such a large spring does not provide satisfactory control feel for a force-feel system or SAS where full time manual control is the objective. Another disadvantage of such systems is that a mechanism (not shown) is required to disconnect the override spring from the flight control system when the autopilot is turned off. Such a mechanism adds weight and must be carefully designed to ensure that it will not fail in a way that it cannot be disconnected from the flight control system. Thus, a mechanical spring is not deemed suitable as a means to provide a parallel force-feel system or SAS.
Tactile feedback enhances the control feel of aircraft. One of the feedbacks to the cockpit-flight-controller that can be used to accomplish improved tactile feel is the position of the cockpit-flight-controller itself. This is commonly done in fly-by-wire systems (not shown) where the flight controller is not mechanically connected to the flight control system. Rather, a flight controller in a fly-by-wire system transmits electrical signals to a servo-actuator which is connected in series, such as the servo-actuator 12 shown in FIG. 1. That is, by contrast to the system 10 shown in FIG. 1, a fly-by-wire system replaces the mechanical link 24 with an electrical connection (i.e., a wire) that transmits a signal directly to the servo-actuator 12. Some fly-by-wire systems replace the feel spring 16 in FIG. 1 with complex algorithms that are intended to improve the tactile feel of the cockpit-flight-controller 20. However, such systems are intended for complex, highly augmented, fly-by-wire aircraft and are not deemed suitable for use on light helicopters.